ACIDS AND BASESACIDS AND BASES

……for it cannot be for it cannot be But I am pigeon-liver’d and lack gall But I am pigeon-liver’d and lack gall To make oppression bitter…To make oppression bitter…HamletHamlet

Learning objectivesLearning objectives

Name and write formulae for common acids and Name and write formulae for common acids and basesbases

Describe acids and bases according to Describe acids and bases according to Arrhenius and Brønsted definitionsArrhenius and Brønsted definitions

Identify conjugate acid-base pairsIdentify conjugate acid-base pairs

Distinguish between strength and concentration Distinguish between strength and concentration in acid-base terminologyin acid-base terminology

Calculate and estimate pH of acids and basesCalculate and estimate pH of acids and bases

Identify buffer solutions and explain actionIdentify buffer solutions and explain action

ACIDS AND BASESACIDS AND BASES

The meaning of acid and base has The meaning of acid and base has changed over the yearschanged over the years

Arrhenius acid is one that Arrhenius acid is one that generatesgenerates protons when dissolved in waterprotons when dissolved in water

Arrhenius base is one that Arrhenius base is one that generatesgenerates hydroxide ions when dissolved in waterhydroxide ions when dissolved in water

Hydronium ion is the active Hydronium ion is the active ingredient of an acidingredient of an acid

Protons do not exist in solutionProtons do not exist in solution

CHCH33COCO22H + HH + H22O = O = HH33OO++ + CH + CH33COCO22--

Vinegar in water produces hydronium ionsVinegar in water produces hydronium ions

Hydroxide ion is the active Hydroxide ion is the active ingredient of a baseingredient of a base

NHNH33 + H + H22O = NHO = NH44++ + + OHOH--

Ammonia, a base, dissolves in water and Ammonia, a base, dissolves in water and produces hydroxide ionsproduces hydroxide ions

NeutralizationNeutralization

The mixing of an acid with a base:The mixing of an acid with a base:

ACID + BASE = SALT + WATERACID + BASE = SALT + WATER

The reaction of carbonic acid (COThe reaction of carbonic acid (CO22 in H in H22O) O)

to give limestone:to give limestone:

HH22COCO33 + Ca(OH) + Ca(OH)22 = CaCO = CaCO33 + 2H + 2H22OO

ACID BASE WATERSALT

The essence of neutralizationThe essence of neutralization

Elimination of the components of acid and Elimination of the components of acid and base by combination to give Hbase by combination to give H22OO

– Reflection of strong O – H bondReflection of strong O – H bond

HH++ + OH + OH-- H H22OO

ACID BASE

BrBrøønsted and Lowrynsted and Lowry

A broader definition of acids and basesA broader definition of acids and bases

In the reaction NHIn the reaction NH33 + HCl = NH + HCl = NH44Cl has all Cl has all

the elements of acid-base neutralization the elements of acid-base neutralization but no Hbut no H22O as would be required in the O as would be required in the

Arrhenius definitionArrhenius definition

Brønsted acid Brønsted acid donatesdonates a proton a proton

Brønsted base Brønsted base acceptsaccepts a proton a proton

Brønsted acidBrønsted acid

HCl + HHCl + H22O = HO = H33OO++ + Cl + Cl--

Brønsted baseBrønsted base

NHNH33 + H + H22O = NHO = NH44++ + OH + OH--

waterwater

NHNH33 + HCl = NH + HCl = NH44++ClCl--

No waterNo water

Substances can be both acids and Substances can be both acids and bases – depends on environmentbases – depends on environment

Note that in one instance HNote that in one instance H22O behaves O behaves

like a base – like a base – acceptingaccepting protons, and in protons, and in another, behaves like an acid – another, behaves like an acid – donatingdonating protonsprotons

HCl + HCl + HH22OO = H = H33OO++ + Cl + Cl--

In presence of an acid HIn presence of an acid H22O is a baseO is a base

NHNH33 + + HH22OO = NH = NH44++ + OH + OH--

In presence of a base HIn presence of a base H22O is an acidO is an acid

The products are themselves acids The products are themselves acids and basesand bases

Equilibrium: solution contains Equilibrium: solution contains mixture of all componentsmixture of all components

Conjugate Conjugate acids and basesacids and bases

Conjugate acid-base pair differs by HConjugate acid-base pair differs by H++

HHAA + + BB ↔ ↔ AA-- + H + HBB++

Conjugate acid-base pair Conjugate acid-base pair

Conjugate baseConjugate acidbase

acid

Strength and concentrationStrength and concentration

Not all acids completely donate the Not all acids completely donate the protons to water molecules in solutionprotons to water molecules in solution

HA + HHA + H22O O A A-- + H + H33OO++

The degree of ionization is described by The degree of ionization is described by strengthstrength

The total number of moles per unit volume The total number of moles per unit volume is described by concentrationis described by concentration

Equilibrium for a weak acidEquilibrium for a weak acid

Measuring acid strength with KMeasuring acid strength with Kaa

– Weak acid has small KWeak acid has small Kaa

– Strong acid has large KStrong acid has large Kaa

AcOHOHHAc 32

HAc

AcOHKa

3

Changing concentration does not Changing concentration does not change strengthchange strength

StrengthStrength refers to degree of ionization: refers to degree of ionization:– Strong is completely ionized (100 %)Strong is completely ionized (100 %)– Weak is partly ionized (1 % - 1:10Weak is partly ionized (1 % - 1:1066))

ConcentrationConcentration refers to number of moles refers to number of moles per unit volumeper unit volume

An acid (or base) can be strong and An acid (or base) can be strong and concentrated, weak and concentrated, concentrated, weak and concentrated, strong and dilute, weak and dilutestrong and dilute, weak and dilute

Relative strength of conjugate pairRelative strength of conjugate pair

The conjugate base of a very strong acid The conjugate base of a very strong acid is itself very weakis itself very weak

The conjugate acid of a very strong base The conjugate acid of a very strong base is itself very weakis itself very weak

AmphotericityAmphotericity

A substance that behaves as an acid and A substance that behaves as an acid and a base is amphoteric. Water is a good a base is amphoteric. Water is a good exampleexample

Ionization of waterIonization of water

Even in pure water, a fraction of the Even in pure water, a fraction of the molecules are ionized and the molecules are ionized and the concentrations of OHconcentrations of OH-- and H and H33OO++ are equal are equal

HH22O + HO + H22O = HO = H33OO++ + OH + OH--

[H[H33OO++] = [OH] = [OH--]]

Concentration

In all aqueous solutions, product of In all aqueous solutions, product of concentrations is a constantconcentrations is a constant

[H[H33OO++][OH][OH--] = constant] = constant

Increasing [HIncreasing [H33OO++] decreases [OH] decreases [OH--] ]

(acidic conditions)(acidic conditions)

Increasing [OHIncreasing [OH--] decreases [H] decreases [H33OO++]]

(basic conditions)(basic conditions)

The pH scale – reduces large The pH scale – reduces large range of numbers to smallrange of numbers to small

In water KIn water KWW = [H = [H33OO++][OH][OH--] = 10] = 10-14-14

pH = - logpH = - log1010[H[H33OO++]]

Range of [HRange of [H33OO++] 10 M – 10] 10 M – 10-14-14 M M

Range of pH -1 to +14 Range of pH -1 to +14 Low pH = acid; high pH = basicLow pH = acid; high pH = basic

pH = 7 = neutralpH = 7 = neutral

Relating pH to [HRelating pH to [H33OO++]]

For pH, take exponent of [HFor pH, take exponent of [H33OO++], change ], change

signsign

– 1010-1-1 M (0.1 M) HCl has pH = M (0.1 M) HCl has pH = 11

– Pure water has [HPure water has [H33OO++] = 10] = 10-7-7 M, pH = M, pH = 77

– Ammonia has [HAmmonia has [H33OO++] =10] =10-11-11 M, pH = M, pH = 1111

Note: change of 1 unit in pH is factor of tenNote: change of 1 unit in pH is factor of ten

Acid strength and pHAcid strength and pH

pH is determined by the ratio of the weak pH is determined by the ratio of the weak acid:conjugate base and the value of Kacid:conjugate base and the value of Kaa

HAc

AcOHKa

3

Ac

KHAcOH a

3

Estimating pHEstimating pH

Estimating pH is often more useful than Estimating pH is often more useful than doing exact calculationsdoing exact calculations

Smaller pH value means larger HSmaller pH value means larger H++ concentrationconcentration

Estimating pHEstimating pH

Smaller pH value means larger HSmaller pH value means larger H++ concentrationconcentration

pH = - logpH = - log1010[H[H33OO++]]

[H[H33OO++] = 10] = 10-4-4

pH = - logpH = - log1010(10(10--44))

= = 44

[H[H33OO++] = ] = 44 x 10 x 10--44

pH = - logpH = - log1010((44 x 10 x 10--44))

= - log= - log101044 - log - log1010(10(10--44))

= = 44 – log – log101044

==44 – – 0.6020.602

= 3.4= 3.4

Concentration 1 x 10Concentration 1 x 10-4-4 M MpH = 4pH = 4

Concentration 4 x 10Concentration 4 x 10-4-4 M MpH = 3.4pH = 3.4

pH and pOHpH and pOH

In any aqueous solution, In any aqueous solution,

[H[H++] x [OH] x [OH--] = 10] = 10-14-14

Obtain [HObtain [H++] given [OH] given [OH--]]

pOH = -logpOH = -log1010[OH[OH--]]

pH + pOH = 14pH + pOH = 14

Relationships between [HRelationships between [H++], pH and [OH], pH and [OH--]]

Buffer solutions control pHBuffer solutions control pH

In biological systems it is often critical for In biological systems it is often critical for stable function to maintain a specific pH stable function to maintain a specific pH environment.environment.

Buffer solutions perform this function – Buffer solutions perform this function – they resist large changes in pH when they resist large changes in pH when either acids or alkalis are addedeither acids or alkalis are added

Buffer solution contains a Buffer solution contains a combination of a weak acid and its combination of a weak acid and its

saltsalt

Mechanism follows Le Chatelier’s Mechanism follows Le Chatelier’s principleprinciple

– Addition of HAddition of H++ converts CH converts CH33COCO22-- to CH to CH33COCO22HH

CHCH33COCO22-- + H + H++ CH CH33COCO22HH

– Addition of OHAddition of OH-- converts CH converts CH33COCO22H to CHH to CH33COCO22--

CHCH33COCO22H + OHH + OH-- CH CH33COCO22-- + H + H22O O

– In general, In general,

Add OHAdd OH- - shifts equilibrium to right (HA shifts equilibrium to right (HA A A--))

HA + HHA + H22O O A A-- + H + H33OO++

Add HAdd H++ shifts equilibrium to left (A shifts equilibrium to left (A-- HA) HA)